1. Field of the Invention
The present invention relates to an image data forming apparatus, and, more particularly, to an image data forming apparatus such as a digital electronic still camera for recording an image signal produced from a solid-state imaging device such as a charge-coupled device, and also to an image data processing method.
2. Description of the Background Art
In general, a solid-state imaging device, such as a charge-coupled device (CCD), has its dynamic range much narrower than the latitude of a silver-halide photosensitive type of negative film. However, recent advances in the semiconductor manufacturing technology and the wide dynamic range image sensor technology, such as multiple-exposure shooting, have made available a wider dynamic range solid-state imaging device.
Nevertheless, when recorded on an image data recording medium or reproduced on a video monitor, the dynamic range of image data generated by such a wide dynamic range solid-state imaging device must be compressed to a range prescribed by a certain standard. Take the video signal compatible with the NTSC standard television system for example. When the video or image signal is reproduced into visible images for display on a monitor, the dynamic range has to be compressed to the signal characteristics of as narrow as 100IRE. This requires gradation control, for example, by giving a knee curve to the gradation characteristics. In addition, when image data is processed in the form of digital data, each pixel of image data is, in general, dealt with in the form of eight bits per channel. Therefore, the dynamic range of image data generated by a solid-state imaging device has to be compressed to this range through gradation control.
In many cases, the NTSC signal and digital signal data are reproduced basically for the purpose of viewing on a video monitor. This means that, when compressing the dynamic range of image signals obtained from a solid-state imaging device, with a reproduction system in which the brightness and the tint appropriate for a picture are already fixed, gradation control is performed such as to produce a xe2x80x9cfinished imagexe2x80x9d, which requires the reproduction system not to make a significant modification but a fine adjustment only. In other words, a conventional image capturing/recording system, which is adapted to record an image signal obtained from a solid-state imaging device, performs most signal processings before recording and records the image signal in the form of a finished or complete image.
From another viewpoint, however, this recording system, adapted to record a finished image, is required to record image data in a form capable of reproducing a picture satisfactory for viewing. This, in turn, requires the image sensor system to perform sophisticated automatic exposure control and automatic white balance control. However, it is not easy to adequately record in a recording dynamic range so restricted as described above a scene, e.g. against the light or in an excess amount of light, which involves a broader dynamic range and in addition a difficulty in calculating an appropriate control value for the automatic exposure control. In addition, because the image data of a finished image is compressed when recorded, the finished image, if unsatisfactory, is difficult to correct.
For example, when a finished image reproduced from digital image data recorded on a recording medium is a little bit over-exposed in its entirety, it is sometimes desired to correct the image toward its under-exposure. It is however not possible to restore highlight data, or part of the image data which has high intensity and was once removed when compressed to a prescribed size, e.g. 8 bits, of recording dynamic range. On such image data, a gradation correction made in the 8-bit dynamic range would give a corrected image whose white part is dimmed or contrast is excessively enhanced.
On the other hand, a large amount of data may be recorded on a silver-halide photosensitive type negative film that has a remarkably wider dynamic range. A print system reproducing a visible image from a silver-halide photosensitive type negative film utilizes this feature of wider dynamic range for printing to correct the image so as to extract only a range of the data appropriate for printing. More specifically, a silver-halide photosensitive type negative film is basically intended to record as much information as possible on a negative film, or a recording medium, so that, when being printed, information only for satisfying the requirements is cut out by correcting the image to form an appropriate finished image. This method is best suited to get an appropriate final image. However, since a silver-halide photosensitive type negative film is so far an intermediate medium, images recorded on a silver-halide negative film cannot be viewed nor therefore used as a finished image.
The ability to record digital image data with a dynamic range as wide as that of a silver-halide photosensitive type negative film and, from this image data, to form and print a finished image, if available, would be very efficient for producing an appropriate final image. However, recording image data with a wide dynamic range and reproducing a finished image from that image data are, in general, incompatible.
For example, assume that there is established an image sensor system with a wide dynamic range capable of covering up to 400% of relative reflectivity (R), or a relative brightness against a gray plate, of a certain part of an objective field. In an imaging system where the imagewise signal from the image sensor system is converted to 8-bit image data for recording onto an image data recording medium, gradation control is performed, when the imagewise signal is converted to eight-bit image data, on the signal which is developed from the imaging sensor system and linear to the amount of light incident to the sensor system. For example, as shown in FIG. 2, the image data of a scene taken against light is distributed as plotted by a curve 101 with much image data in a higher relative reflectivity area. More specifically, for a scene including much highlight data as such, effective data may be recorded up to almost the maximum gradation level xe2x80x9c255xe2x80x9d represented by eight bits. The gradation control curve therefor is plotted as indicated by a reference numeral 103.
On the other hand, the gradation of the image data of a scene taken in the for light or in the shade is plotted as indicated by a reference numeral 105 in FIG. 3, with more image data in areas with a lower relative reflectivity. Therefore, performing gradation control for a scene with less highlight data in the eight-bit gradation range makes it difficult to use the entire range efficiently. In addition, because the relative reflectivity of a main object, such as a man or woman, is generally 100% or lower, the application of the same gradation control curve 103 would produce as a whole a dark image that is not practically viewed.
Japanese patent laid-open publication No. 131718/1995 discloses an image composing apparatus that shoots one scene in different exposure amounts in a single shooting operation and then composes the data of the thus obtained plurality of images to form a picture of wider dynamic range. More specifically, this image composing apparatus combines two types of image data: one is standard image data shot with a standard exposure and the other is non-standard image data shot with a non-standard exposure, for example, using a high-speed shuttering. The image composing circuit of the apparatus is adapted to determine from both images the common areas whose intensity level falls in a predetermined range and then shift the intensity levels of the common areas so as to match with each other. The circuit in turn replaces the areas of the non-standard image which have the intensity level falling in the predetermined range for the areas of the standard image which correspond to the former areas of the non-standard image and have the intensity outside the predetermined range. By doing so, white- or black-filled areas in one image are replaced with the corresponding areas in the other. This known image composing apparatus is configured so that the data of two different images may be generated by shooting the same scene, each with its own exposure.
This known image composing apparatus teaches us how to form a finished image using wide dynamic range image data. When the user wants to correct the finished image by himself or herself or during reproduction on the subsequent reproducing apparatus, the composing apparatus requires the finished image per se to be corrected. In other words, regardless of the fact that wide dynamic range image data has been captured by the imaging system, part of the image data which is not involved in the finished image cannot be used at a later time.
It is therefore an object of the present invention to provide an image data forming apparatus and an image data processing method capable of recording image data with a wide dynamic range onto an image data recording medium and reproducing a finished image from the thus recorded image data.
The image data forming apparatus according to the present invention comprises an input circuit for receiving a first image signal forming an image; a range separator connected to the input circuit for separating the first image signal into a second and a third image signal, the second image signal corresponding to a first range of a dynamic range which the first image signal has and being suitable for forming a finished image of the first image signal, the third image signal corresponding at least to a second range other than the first range; and an output circuit connected to the range separator for associating the second image signal with the third image signal as the image and outputting the second and third image signals.
Preferably, the output circuit may comprise a recording memory for recording therein the second image signal and the third image signal.
The input circuit may preferably comprise a solid-state imaging device for imaging a subject field as the image and generating, as the first image signal, the image signal representing the subject field.
According to the present invention, the image reproducer comprises an input circuit for receiving a first image signal corresponding to a first range suitable for forming a finished image of an image and a second image signal corresponding at least to a second range other than the first range, the first and second image signals being associated with each other as the image; a range combiner connected to the receiving circuit for developing and combining the first and second image signals on a common axis and for generating a third image signal having a dynamic range including the first and second ranges; and an image output circuit for reproducing the image from the third image signal.
Preferably, the receiving circuit may comprise a recording memory recording therein the first and second image signals associated with each other; and a reading circuit reading out the first and second image signals associated with each other as the image from the recording memory.
According to the present invention, the image data processing method comprises the steps of receiving a first image signal forming an image; separating the first image signal into a second and a third image signal, the second image signal corresponding to a first range of a dynamic range which the first image signal has and being suitable for forming a finished image of the first image signal, the third image signal corresponding at least to a second range other than the first range; and associating the second image signal with the third image signal as the image and outputting the second and third image signals.
Preferably, the image data processing method may further comprise the step of recording the second and third image signals associated with each other and output on a recording medium.
The image data processing method may further comprise the steps of reading out the second and third signals associated with each other as the image from the recording medium; developing and combining the second and third image signals read out on a common axis and forming the first image signal having the dynamic range; and reproducing the image from the first image signal formed.
The image data processing method may further comprise the steps of reading out the second image signal from the recording medium; and visualizing the image in a form of finished image from the second image signal read.
In this way, the finished image may be formed and recorded from wide dynamic range image data. At the same time, image data not used for forming the finished image is recorded so as to enable a reproducer system to efficiently utilize those data.
In this specification, the term xe2x80x9ca wide dynamic range imaging devicexe2x80x9d refers to an imaging device with several kinds of photosensitive cells which are different in sensitivity from each other.